The invention relates generally to systems for storing liquid cryogens, and, more particularly, to a system for detecting overflow of a liquid cryogen storage tank.
Liquid cryogen storage tanks are typically found at hospitals, factories, and other facilities that feature cryogenic use devices. A typical cryogen storage tank features double wall construction with input valves. To refill such a tank, truck transporting a liquid cryogen (such as nitrogen, hydrogen or liquid natural gas) is connected to the input valves of the storage tank. The storage tank is then refilled with the liquid cryogen from the transport truck. It is necessary to monitor the level of liquid cryogen while refilling the tank to prevent an overflow which would result in a lack of vapor space in the tank. A vapor space, i.e. head space, is necessary to provide room for liquid expansion as the tank absorbs heat.
Normally, a xe2x80x9cTrycockxe2x80x9d system is used to detect the refill level in the storage tanks. The Trycock system consists of a dip tube positioned in the top part of the tank. The tube extends from inside the head space of the tank to the outside of the tank by way of a pipe. If liquid raises to the level of the dip-tube, it will flow through the tube and exit thereby indicating the liquid level is at the dip-tube.
Flammable gases, such as hydrogen or liquid natural gas (LNG), exiting the tank must be directed to a vent stack that directs the gas to a safe area for discharge. It is harmful to the environment to discharge the cryogen directly to the atmosphere. Thus, there exists a need for a device that can indicate the presence of liquid in the exit pipe or the vent stack.
The prior art discloses systems with devices that measure the level of liquid cryogen in a tank to determine when the tank is full. These systems, however, suffer from disadvantages. An example is a system having a pipe that places the head space of the tank in communication with a vent stack outside of the tank. The vent stack is a vertical pipe that is open at the top and requires liquid cryogen to travel against the force of gravity before exiting the stack. This system prolongs the time it takes for the liquid cryogen to enter the atmosphere. As such, liquid is permitted to flow to the stack and the stack is monitored for visual signs of liquid entering the stack such as condensation appearing on the stack or the presence of liquid at the top of the stack.
The liquid cryogen will continue to flow through the pipe and up the vent stack until the input valve is manually closed by the operator. If there is no obvious signal, the operator might not be aware that liquid cryogen has started to flow through the pipe. The operator must constantly monitor the stack for condensation or the presence of liquid. The problem with this method of detecting is that an operator may not be constantly watching the stack to see the condensation or liquid and might not be able to shut off the valve in a timely manner.
A modified version of the above system includes a gas thermometer in communication with the pipe. This system requires the gas thermometer to be charged with a gas that responds to the presence of liquid in the pipe. Gas thermometers charged with neon or hydrogen have proven successful, but sometimes encounter difficulties when exposed to LNG temperatures. This system is also not a fail safe method if the thermometer is not continuously monitored by the operator. Failure to timely notice the change in temperature that signifies the change in phase (gas to liquid) traveling through the pipe can lead to overfilling the tank.
Another system used to detect overflow has a tube in the head space of the tank that extends outside of the tank and uses optical sensors, i.e. light beams. The light beam passes through the pipe. Once the liquid cryogen begins to travel through the pipe the light beam is interrupted. This system is not ideal because it is expensive and the signal still must be continuously watched by the operator. By the time the operator is aware of the signal, it is often too late and the tank has been over filled.
Accordingly, it is an object of the present invention to provide a device to alert an operator that the storage tank has reached a predetermined fill level of liquid cryogen.
It is another object of the present invention to provide a device to detect overflow in a liquid cryogen tank that is economical, simple and does not require constant operator attention.
It is another object of the present invention to provide a device to detect overflow in a liquid cryogen tank that is easy to install in existing storage systems.
The present invention is directed to a system for storing cryogenic fluid. The cryogenic fluid is stored in an insulated tank that includes an input line for receiving cryogenic fluid and a head space for storing cryogenic vapor. The inlet line is in communication with a valve that receives the liquid cryogen from a delivery truck. An outlet pipe is in communication with the head space of the tank. A device for detecting overflow of the tank is in communication with the outlet pipe. The device produces an audible signal responsive to a change in state of the cryogenic fluid passing through the outlet pipe.
In another aspect of this invention, the overflow detecting device includes a housing that defines an interior chamber with a tab secured over an opening in the housing. The device is connected to the outlet pipe of the tank. The device produces an audible signal when cryogenic vapor passes through the outlet pipe, but no signal when liquid cryogen passes through the outlet pipe.
Alternatively or additionally, a visual back-up device may be used with the present system. A closed loop pipe can be.;connected to the outlet pipe so that when liquid cryogen flows through the closed loop pipe, a frost develops on the pipe signifying to the operator that liquid cryogen has left the tank and the input valve needs to be closed.
The following detailed description of embodiments of the invention, taken in conjunction with the appended claims and accompanying drawings, provide a more complete understanding of the nature and scope of the invention.